cmd/compile/internal/ssa: inline memmove with known size

Replace calls to memmove with known (constant) size, with OpMove.
Do it only if it is safe from aliasing point of view.
Helps with code like this:

append(buf,"const str"...)

In strconv this provides nice benefit:
Quote-6                                   731ns ± 2%   647ns ± 3%  -11.41%  (p=0.000 n=10+10)
QuoteRune-6                               117ns ± 5%   111ns ± 1%   -4.54%  (p=0.000 n=10+10)
AppendQuote-6                             475ns ± 0%   396ns ± 0%  -16.59%  (p=0.000 n=9+10)
AppendQuoteRune-6                        32.0ns ± 0%  27.4ns ± 0%  -14.41%  (p=0.000 n=8+9)

Change-Id: I7704f5c51b46aed2d8f033de74c75140fc35036c
Reviewed-on: https://go-review.googlesource.com/54394
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/asm_test.go

@@ -1191,6 +1191,36 @@ var linuxAMD64Tests = []*asmTest{
 		`,
 		pos: []string{"\tANDQ\t\\$4095,"},
 	},
+	{
+		// Test that small memmove was replaced with direct movs
+		fn: `
+                func $() {
+                       x := [...]byte{1, 2, 3, 4, 5, 6, 7}
+                       copy(x[1:], x[:])
+                }
+		`,
+		neg: []string{"memmove"},
+	},
+	{
+		// Same as above but with different size
+		fn: `
+                func $() {
+                       x := [...]byte{1, 2, 3, 4}
+                       copy(x[1:], x[:])
+                }
+		`,
+		neg: []string{"memmove"},
+	},
+	{
+		// Same as above but with different size
+		fn: `
+                func $() {
+                       x := [...]byte{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}
+                       copy(x[1:], x[:])
+                }
+		`,
+		neg: []string{"memmove"},
+	},
 }
 
 var linux386Tests = []*asmTest{
@@ -1322,6 +1352,26 @@ var linux386Tests = []*asmTest{
 		`,
 		pos: []string{"\tANDL\t\\$4095,"},
 	},
+	{
+		// Test that small memmove was replaced with direct movs
+		fn: `
+                func $() {
+                       x := [...]byte{1, 2, 3, 4, 5, 6, 7}
+                       copy(x[1:], x[:])
+                }
+		`,
+		neg: []string{"memmove"},
+	},
+	{
+		// Same as above but with different size
+		fn: `
+                func $() {
+                       x := [...]byte{1, 2, 3, 4}
+                       copy(x[1:], x[:])
+                }
+		`,
+		neg: []string{"memmove"},
+	},
 }
 
 var linuxS390XTests = []*asmTest{